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Program: Inheritance03.java
// Learning Object Inheritance03
// dynamic dispatchingclass Particle {
int position;
Particle(int p) { position = p;
}
void newPosition(int delta) { position = position + delta;
}}
class AParticle extends Particle {
double spin;
AParticle(int p, double s) { super(p);
spin = s; }
void newPosition(int delta) {
if (spin < delta)
position = position + delta; }
}
class BParticle extends Particle { int charge;
BParticle(int p, int c) {
super(p); charge = c;
}}
class CParticle extends BParticle {
boolean strange;
CParticle(int p, int c, boolean s) { super(p, c);
strange = s; }
void newPosition(int delta) {
if (strange) position = position * charge;
}}
class Inheritance03 {
public static void main(/*String[] args*/) {
Particle p = new Particle(10); AParticle a = new AParticle(20, 2.0);
BParticle b = new BParticle(30, 3); CParticle c = new CParticle(40, 4, true);
p.newPosition(10);
int pPosition = p.position; p = a;
p.newPosition(10); int aPosition = p.position;
p = b; p.newPosition(10);
int bPosition = p.position; p = c;
p.newPosition(10); int cPosition = p.position;
}}
p ,
and then the method
newPosition is invoked using
p . The modified
value of
position is assigned to a variable.p invokes the method defined in class
Particle .a to
p , check that the call invokes the
method defined in the class
AParticle ; this method overrides the method declared
in class
Particle . Although the type of
p is class
Particle , it holds a reference to an object whose type is
class
AParticle so the method of that class is called.Particle has become garbage.b to
p , check that the call invokes the
method defined in the superclass
Particle ; since the method was
not overridden in
BParticle , the method called is the one inherited from the
superclass.c to
p , check that the call invokes the
method defined in the class
CParticle ; this method overrides the method declared
in class
Particle . Although the type of
p is class
Particle , it holds a reference to an object whose type is
class
CParticle so the method of that class is called.
Exercise Add an assignment of
c to
b and call
b.newPosition(10) . What is the value now of
b.position ?
Concept A variable of the type of a class can reference an object of the type of a subclass , but this variable cannot be used to access fields declared in the subclass.Nevertheless, the object “remembers” its type, even if it is assigned to a variable of the type of its superclass, and the type can be “recovered”by casting to a variable of the type of the subclass. This is called downcasting because the cast is “down” the derivation hierarchy.
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